Gain control circuits have been used for controlling the level of gain applied to a received signal or a signal to be transmitted. In many instances it is desirable to maintain a consistent signal level, over different operating conditions. For example in a cellular communication environment, it is desirable to transmit a signal having a signal level sufficient for being received at all locations within a cell, but not so strong as to cause interference in nearby cells. Often times the optimum signal level is determined by the size of the cell or the physical location of the communication antenna within the cell.
Gain control, rather than output power control, is needed if the total transmitted signal contains multiple voice traffic channels which are selectively enabled, causing the composite signal power to vary. Gain control will keep the level of each individual voice channel constant when the channel is enabled, while allowing variation of the composite signal level. Gain can also vary dependent upon other factors such as variations in signal frequency, temperature, and input signal levels, and therefore gain control circuits have been used to account for changes of these types.
Typically a gain control circuit will measure the operational signal levels by sampling the signal levels at the input and the output. In many RF amplifiers this is performed by using Schottky diodes. Unfortunately these diodes do not function reliably below certain power levels, because temperature compensation becomes more difficult and voltage levels become too low to overcome offset voltages and noise in the associated circuitry.
When very low signal levels or no signal levels are detected, inherent voltage errors in the control circuitry, which can result from detector bias drift, op amp offset voltages and currents, etc., becomes a significant component of the signals used to determine the value of the control signal. This causes the gain to drift to levels not fully reflective of the actual signal levels. At these signal levels the gain control circuit is ineffective for providing the necessary gain adjustments, resulting in a reduced dynamic range.
When no signal is detected or no signal is present, the inherent error voltages in the control circuitry will cause the gain of the amplifier to drift, often times adjusting to its maximum level. Subsequently when the input signal reaches a level no longer dominated by the error voltages, the signal will be initially amplified a maximum amount, until the control loop stabilizes the gain, which can cause the generation of a signal to be transmitted having a signal level which exceeds acceptable operating levels. This could result in interference, as well as damage to the present or subsequent amplifier stages. Therefore a gain control circuit having an expanded dynamic range over which the gain is stable would be beneficial.